I will ask for what I want
Partners are not mind readers. No matter how intimately my husband knows me, I can't expect him to interpret "I'm fine" as actually meaning "I need you to ask me if I'm OK three more times because I'm feeling sad, but I'm also vulnerable and don't know how to give words to my emotions right now."
Waiting for someone to guess why you're upset is only prolonging your bad mood, and will probably just frustrate your partner. I try to remind myself to speak up when I need something from my husband, whether it's more help around the house or extra encouragement when I'm stressed.
Do I always do this? Definitely not; I'm the master of "I'm fine." But the more I work on this and tell my partner when I need something, the less I will feel neglected, ignored, or lonely.